In order to allow an ephemeral, or otherwise pristine image to
receive some configuration, cloud-init will read a url directed by
the kernel command line and proceed as if its data had previously existed.

This allows for configuring a meta-data service, or some other data.

Note, that usage of the kernel command line is somewhat of a last resort,
as it requires knowing in advance the correct command line or modifying
the boot loader to append data.

For example, when 'cloud-init start' runs, it will check to
see if if one of 'cloud-config-url' or 'url' appear in key/value fashion
in the kernel command line as in:
   root=/dev/sda ro url=http://foo.bar.zee/abcde

Cloud-init will then read the contents of the given url. 
If the content starts with '#cloud-config', it will store
that data to the local filesystem in a static filename 
'/etc/cloud/cloud.cfg.d/91_kernel_cmdline_url.cfg', and consider it as
part of the config from that point forward.

If that file exists already, it will not be overwritten, and the url parameters
completely ignored.

Then, when the DataSource runs, it will find that config already available.

So, in able to configure the MAAS DataSource by controlling the kernel
command line from outside the image, you can append:
   url=http://your.url.here/abcdefg
or
   cloud-config-url=http://your.url.here/abcdefg

Then, have the following content at that url:
   #cloud-config
   datasource:
    MAAS:
      metadata_url: http://mass-host.localdomain/source
      consumer_key: Xh234sdkljf
      token_key: kjfhgb3n
      token_secret: 24uysdfx1w4

Notes:
 * Because 'url=' is so very generic, in order to avoid false positives,
   cloud-init requires the content to start with '#cloud-config' in order
   for it to be considered.
 * The url= is un-authed http GET, and contains credentials
   It could be set up to be randomly generated and also check source 
   address in order to be more secure